Don't ever get a greyhound for protection, though a big dog may put some people off. They will let anyone in the house and they only bark at squirrels.
They will only wake up if you open the fridge door but still won't get up to get cheese, you have to take it to their bed. They are completely loony but they love their humans with all of their big hearts (literally big, lungs are huge too) if you are down or poorly they will sit with you in total empathy, they will make you laugh with their antics, they are very kind dogs which is remarkable considering how they are treated by humans.
If you need a guard dog choose a good one but please support the closing down of greyhound racing sites and chuck some money at your nearest greyhound rescue, you could even pop in after all you can have a guard dog and a greyhound.